Most of the members of this series are remarkable on account of the great concentration of the nerve-centres. This is extreme in Rhizotrogus, where there are only two great ganglia, viz. the supra-oesophageal and a great ganglion situate in the thorax, and consisting of the conjoined infra-oesophageal, thoracic, and abdominal ganglia. According to Brandt[[80]] there are several distinct forms of concentration in the series; the Lucanidae only participate in it to the extent that the perfect Insects exhibit fewer ganglia than the larvae; the latter possess two cephalic, three thoracic, and eight abdominal ganglia, while the perfect Insect has the abdominal ganglia reduced in number to six, and they are placed partially in the thorax. The diminution in number takes place in this case by the amalgamation of the first two abdominal with the last thoracic ganglia.
Fig. 86—View of one side of meso- and metathorax of a Passalid larva from Borneo showing the stridulating organs. a, b, Portions of the metathorax; c, coxa of 2nd leg; d, striate or stridulating area thereon; e, basal part of femur of middle leg; f, hairs with chitinous process at base of each; g, the diminutive 3rd leg modified for scratching the striated area. × 20.
Fam. 1. Passalidae.—Labrum large, mobile; mentum deeply cut out in the middle for the accommodation of the ligula; the lamellae of the antenna brought together by the curling up of the antenna. The elytra entirely cover the dorsal surface of the abdomen. There are four or five hundred species of this family known; they are usually shining-black, unattractive beetles, of large size, and are abundant in the decaying wood of tropical forests. They are quite absent from Europe, and there is only one species in the United States of North America. The larvae are very interesting, from the fact that they appear to have only four legs. This arises from the posterior pair being present only as very short processes, the function of which is to scrape striated areas on the preceding pair of legs and so produce sound. In the species figured (Fig. 86) this short leg is a paw-like structure, bearing several hard digits; but in other species it is more simple, and without the digits. The perfect Insect has no sound-producing organs, and it is very remarkable therefore to find the larvae provided with highly-developed stridulatory structures. No auditory organ is known, unless the peculiar spiracles be such.
Fig. 87—Head and prothorax of forms of the male of a stag-beetle; Homocoderus mellyi (Africa). A, Large, B, intermediate, C, small forms. (From a photograph by R. Oberthür.)
Fam. 2. Lucanidae (Stag-beetles).—Labrum indistinct, fixed; mentum not excised; antennae not curled in repose, with but little coadaptation of the terminal joints; the elytra entirely cover the dorsal surface of the abdomen. The Stag-beetles are well known on account of the extraordinary development of the mandibles in the male sex, these organs being in some cases nearly as long as the whole of the rest of the Insect, and armed with projections or teeth that give the Insects a most formidable appearance. So far as we have been able to discover, these structures are put to very little use, and in many cases are not capable of being of service even as weapons of offence. The males are usually very much larger than the females, and are remarkable on account of the great variation in the stature of different individuals of the same species; correlative with these distinctions of individual size we find extreme differences in the development of the head and mandibles. Moreover, the small male specimens exhibit not merely reductions in the size of the mandibles, but also show considerable differences in the form of these parts, due, in some cases, apparently to the fact that only when a certain length of the mandible is attained is there any development of certain of the minor projections: in other cases it is not possible to adopt this view, as the small mandibles bear as many projections as the large forms do, or even more. In each species these variations fall, in the majority of cases, into distinct states, so that entomologists describe them as "forms," the largest developments being called teleodont, the smallest priodont; the terms mesodont and amphiodont being applied to intermediate states. Leuthner, who has examined many specimens, states that in Odontolabis sinensis, no intermediates between the teleodont and mesodont forms occur, and as the two forms are very different they are liable to be mistaken for distinct species.
There are at present between 500 and 600 species of stag-beetles known; the Indo-Malayan and Austro-Malayan regions being richest in them. Australia possesses many remarkable and aberrant forms. In the Ceratognathini—a group well represented in New Zealand as well as in Australia—the structure of the antennae is like that of the Scarabaeidae, rather than of the Lucanidae. The most aberrant form known is, however, our common Sinodendron cylindricum; this departs in numerous features from other Lucanidae, and instead of the mandibles of the male being more largely developed, there is a horn on the head; it is very doubtful whether this Insect should be allowed to remain in the family. Little is known of the habits and development of Lucanidæ, except in the case of three or four species that are common in Europe.
Fig. 88—Sinodendron cylindricum. A, Larva; B, pupa. New Forest.